Generation of 3D Fibroblast-Derived Extracellular Matrix and Analysis of Tumor Cell-Matrix Interactions and Signaling

Fibroblasts are the major producers of the extracellular matrix and regulate its organization. Aberrant signaling in diseases such as fibrosis and cancer can impact the deposition of the matrix proteins, which can in turn act as an adhesion scaffold and signaling reservoir promoting disease progression. To study the composition and organization of the extracellular matrix as well as its interactions with (tumor) cells, this protocol describes the generation and analysis of 3D fibroblast-derived matrices and the investigation of (tumor) cells seeded onto the 3D scaffolds by immunofluorescent imaging and cell adhesion, colony formation, migration, and invasion/transmigration assays.

Physically-based structural modeling of a typical regenerative tissue analog bridges material macroscale continuum and cellular microscale discreteness and elucidates the hierarchical characteristics of cell-matrix interaction

This paper presents a comprehensive physically-based structural modelling for the passive and active biomechanical processes in a typical engineered tissue - namely, cell-compacted collagen gel. First, it introduces a sinusoidal curve analog for quantifying the mechanical response of the collagen fibrils and a probability distribution function of the characteristic crimp ratio for taking into account the fibrillar geometric entropic effect. The constitutive framework based on these structural characteristics precisely reproduces the nonlinearity, the viscoelasticity, and fairly captures the Poisson effect exhibiting in the macroscale tensile tests; which, therefore, substantially validates the structural modelling for the analysis of the cell-gel interaction during collagen gel compaction. Second, a deterministic molecular clutch model specific to the interaction between the cell pseudopodium and the collagen network is developed, which emphasizes the dependence of traction force on clutch number altering with the retrograde flow velocity, actin polymeric velocity, and the deformation of the stretched fibril. The modelling reveals the hierarchical features of cellular substrate sensing, i.e. a biphasic traction force response to substrate elasticity begins at the level of individual fibrils and develops into the second biphasic sensing by means of the fibrillar number integration at the whole-cell level. Singular in crossing the realms of continuum and discrete mechanics, the methodologies developed in this study for modelling the filamentous materials and cell-fibril interaction deliver deep insight into the temporospatially dynamic 3D cell-matrix interaction, and are able to bridge the cellular microscale and material macroscale in the exploration of related topics in mechanobiology.

Raman spectroscopy assisted biochemical evaluation of L929 fibroblast cells on differentially crosslinked gelatin hydrogels

Biochemical evaluation of cell-matrix interaction using conventional labelling techniques often possesses limitations due to dye entrapment. In contrast, Raman spectroscopy guided approach offers label-free determination of cell-matrix biochemistry. Herein, gelatin (Gel) matrices modified with 1-Ethyl-3-(3-dimethylaminopropyl) carbodiimide/ N-Hydroxysuccinimide (EDC/NHS) and glutaraldehyde (GTA) was used as standards for comparative evaluation. Raman spectroscopy was deployed as a label-free approach to investigate interaction of cells with Gel hydrogels. Raman-based approach assisted in evaluation of cell-matrix interactions by identifying key biomolecular signatures retrospecting the fact that L929 fibroblast cells portrayed excellent growth and proliferation kinetics in crosslinked Gel as compared to its bare counterpart. EDC crosslinked hydrogels exhibited superior cell proliferation than its GTA counterparts. Cell proliferation on differentially crosslinked gel was also confirmed using standard MTT Assay and Rhodamine-DAPI staining thus corroborating the fact that Raman spectroscopy can be deployed as a superior label-free alternative towards real-time determination of cell proliferation and growth.

Distinctive roles of fibrillar collagen I and collagen III in mediating fibroblast-matrix interaction: A nanoscopic study

One major goal in tissue engineering is to create functional materials, mimicking scaffolds in native tissues, to modulate cell function for tissue repair. Collagen is the most abundant structural protein in human body. Though collagen I (COLI) and collagen III (COLIII) are the predominant collagen types in connective tissues and they form stable hybrid fibrils at varied ratios, cell responses to the hybrid matrices are underinvestigated. In this work, we aim to explicate the distinctive roles of COLI and COLIII in fibroblast activation. Unidirectionally aligned COLI, COLIII and COLI-COLIII hybrid nanofibrils were generated via epitaxial growth of collagen on mica. AFM analyses revealed that, with the increase of COLI/COLIII ratio, the fibril width and stiffness increased and the binding affinity of cells to the matrix decreased. A hybrid matrix was found to activate fibroblasts the most effectively, characterized by extensive cell polarization with rigid stress fiber bundles and high α-SMA expression, and by the highest-level of collagen synthesis. It is ascribed to the fine balance between biochemical and biophysical cues achieved on the hybrid matrix. Thus, matrices of aligned COLI-COLIII hybrid fibrils and their derived multifunctional composites can be good candidates of implantation scaffolds for tissue regeneration.

Imaging of Human Cancer Cells in 3D Collagen Matrices

Research on cell migration and interactions with the extracellular matrix (ECM) was mostly focused on 2D surfaces in the past. Many recent studies have highlighted differences in migratory behaviour of cells on 2D surfaces compared to complex cell migration modes in 3D environments. When embedded in 3D matrices, cells constantly sense the physicochemical, topological and mechanical properties of the ECM and adjust their behaviour accordingly. Changes in the stiffness of the ECM can have effects on cell morphology, differentiation and behaviour and cells can follow stiffness gradients in a process called durotaxis. Here we introduce a detailed protocol for the assembly of 3D matrices consisting of collagen I/fibronectin and embedding cells for live cell imaging. Further, we will show how the matrix can be stiffened via non-enzymatic glycation and how collagen staining with fluorescent dyes allows simultaneous imaging of both matrix and cells. This approach can be used to image cell migration in 3D microenvironments with varying stiffness, define cell-matrix interactions and the cellular response to changing ECM, and visualize matrix deformation by the cells.

Evaluation of alginate modification effect on cell-matrix interaction, mechanotransduction and chondrogenesis of encapsulated MSCs

Mesenchymal stem cells (MSCs) are promising cell candidates for cartilage regeneration. Furthermore, it is important to control the cell-matrix interactions that have a direct influence on cell functions. Providing an appropriate microenvironment for cell differentiation in response to exogenous stimuli is a critical step towards the clinical utilization of MSCs. In this study, hydrogels consisted of different proportions of alginates that were modified using gelatin, collagen type I and arginine-glycine-aspartic acid (RGD) and were evaluated regarding their effects on mesenchymal stem cells. The effect of applying hydrostatic pressure on MSCs encapsulated in collagen-modified alginate with and without chondrogenic medium was evaluated 7, 14 and 21 days after culture, which is a comprehensive evaluation of chondrogenesis in 3D hydrogels with mechanical and chemical stimulants. Alcian blue, safranin O and dimethyl methylene blue (DMMB) staining showed the chondrogenic phenotype of cells seeded in the collagen- and RGD-modified alginate hydrogels with the highest intensity after 21 days of culture. The results of real-time PCR for cartilage-specific extracellular matrix genes indicated the chondrogenic differentiation of MSCs in all hydrogels. Also, the synergic effects of chemical and mechanical stimuli are indicated. The highest expression levels of the studied genes were observed in the cells embedded in collagen-modified alginate by loading after 14 days of exposure to the chondrogenic medium. The effect of using IHP on encapsulated MSCs in modified alginate with collagen type I is equal or even higher than using TGF-beta on encapsulated cells. The results of immunohistochemical assessments also confirmed the real-time PCR data.

Substrate mechanics dictate cell-cell communication by gap junctions in stem cells from human apical papilla

It is recognized that the interaction between cells and their physical microenvironment plays a fundamental role in controlling cell behaviors and even in determining cell fate. Any change in the physical properties of the extracellular matrix (ECM), such as its topography, geometry, and stiffness, controls this interaction. In the current study, we revealed a potent interconnection between the cell-matrix interaction and cell-cell communication that is mediated by interface stiffness, and elucidated this process in stem cells from human apical papilla (hSCAPs) in terms of mechanosensing, mechanotransduction, and gap junction-mediated cell-cell communication. We first fabricated polydimethylsiloxane (PDMS) substrates with the same topography and geometry but different stiffnesses and found that the cell morphology of the hSCAPs actively changed to adapt to the difference in substrate stiffness. We also found that the hSCAPs secreted more fibronectin in response to the stiff substrate. The focal adhesion plaques were changed by altering the expression of focal adhesion kinase (FAK) and paxillin. The FAK and paxillin bound to connexin 43 and, as a result, altered the gap junction formation. By performing a Lucifer yellow transfer assay, we further confirmed that the interface stiffness mediated cell-cell communication in living hSCAPs through changes in gap junction tunnels. The intrinsic mechanism that mediated cell-cell communication by extracellular stiffness show the great influence of the interaction between cells and their external physical microenvironment and stress the importance of microenvironmental mechanics in organ development and diseases. STATEMENT OF SIGNIFICANCE: Biochemical factors could direct cell behaviors such as cell proliferation, migration, differentiation, cell cycling and apoptosis. Likewise, biophysical factors could also determine cell behaviors in all biological processes. In the current study, we revealed a potent interconnection between the cell-matrix interaction and cell-cell communication by elucidating the whole process from cell mechanosensing, mechanotransduction to gap junction-mediated cell-cell communication. This process occurs in a collective of cells but not in that of a single cell. Biophysical properties of ECM induced cell-to-cell communication indicates the importance of microenvironmental mechanics in organ development and diseases. These findings should be of great interest in all biological fields, especially in biomaterials - cell/molecular biology involved in the interactions between the cell and its matrix.

A theoretical model of collective cell polarization and alignment

Collective cell polarization and alignment play important roles in tissue morphogenesis, wound healing and cancer metastasis. How cells sense the direction and position in these processes, however, has not been fully understood. Here we construct a theoretical model based on describing cell layer as a nemato-elastic medium, by which the cell polarization, cell alignment and cell active contraction are explicitly expressed as functions of components of the nematic order parameter. To determine the order parameter we derive two sets of governing equations, one for the force equilibrium of the system, and the other for the minimization of the system's free energy including the energy of cell polarization and alignment. By solving these coupled governing equations, we can predict the effects of substrate stiffness, geometries of cell layers, external forces and myosin activity on the direction- and position-dependent cell aspect ratio and cell orientation. Moreover, the axisymmetric problem with cells on a ring-like pattern is solved analytically, and the analytical solution for cell aspect ratio are governed by parameter groups which include the stiffness of the cell and the substrate, the strength of myosin activity and the external forces. Our predictions of the cell aspect ratio and orientation are generally comparable to experimental observations. These results show that the pattern of cell polarization is determined by the anisotropic degree of active contractile stress, and suggest a stress-driven polarization mechanism that enables cells to sense their spatial positions to develop direction- and position-dependent behavior. This, in turn, sheds light on the ways to control pattern formation in tissue engineering for potential biomedical applications.

Loss of cytoskeleton protein ADD3 promotes tumor growth and angiogenesis in glioblastoma multiforme

Adducin 3 (ADD3) is a crucial assembly factor in the actin cytoskeleton and has been found to be aberrantly expressed in various cancers, including glioblastoma multiforme (GBM). It has previously been studied in array-based studies with controversial findings as to its functional role in glioma. In microarray analyses of 452 glioma specimens, we found significant downregulation of ADD3 in GBM, but not in less malignant gliomas, compared to normal brain tissue, which suggests that its downregulation might underlie critical events during malignant progression. We also found that ADD3 was functionally dependent on cell-matrix interaction. In our in vivo study, the proliferative and angiogenic capacity of ADD3-depleted GBM cells was promoted, possibly through PCNA, while p53 and p21 expression was suppressed, and pro-angiogenic signals were induced through VEGF-VEGFR-2-mediated activation in endothelial cells. With correlative in vitro, in vivo, and clinical data, we provide compelling evidence on the putative tumor-suppressive role of ADD3 in modulating GBM growth and angiogenesis. As a preclinical study, our research offers a better understanding of the pathogenesis of glioma malignant progression for the benefit of future investigations.

Substrate mechanics controls adipogenesis through YAP phosphorylation by dictating cell spreading

The mechanoregulated proteins YAP/TAZ are involved in the adipogenic/osteogenic switch of mesenchymal stem cells (MSCs). MSC fate decision can be unbalanced by controlling substrate mechanics, in turn altering the transmission of tension through cell cytoskeleton. MSCs have been proposed for orthopedic and reconstructive surgery applications. Thus, a tight control of their adipogenic potential is required in order to avoid their drifting towards fat tissue. Substrate mechanics has been shown to drive MSC commitment and to regulate YAP/TAZ protein shuttling and turnover. The mechanism by which YAP/TAZ co-transcriptional activity is mechanically regulated during MSC fate acquisition is still debated. Here, we design few bioengineering tools suited to disentangle the contribution of mechanical from biological stimuli to MSC adipogenesis. We demonstrate that the mechanical repression of YAP happens through its phosphorylation, is purely mediated by cell spreading downstream of substrate mechanics as dictated by dimensionality. YAP repression is sufficient to prompt MSC adipogenesis, regardless of a permissive biological environment, TEAD nuclear presence or focal adhesion stabilization. Finally, by harnessing the potential of YAP mechanical regulation, we propose a practical example of the exploitation of adipogenic transdifferentiation in tumors.

ECM signaling in cartilage development and endochondral ossification

During cartilage development chondrocytes undergo a multi-step process characterized by consecutive changes in cell morphology and gene expression. Cell proliferation, polarity, differentiation, and migration are influenced by chemical and mechanical signaling between the extracellular matrix (ECM) and the cell. Several structurally diverse transmembrane receptors such as integrins, discoidin domain receptor 2 (DDR 2), and CD44 mediate the crosstalk between cells and their ECM. However, the contribution of cell-matrix interactions during early chondrogenesis and further cartilage development through cell receptors and their signal transduction pathways is still not fully understood. Determination of receptor signaling pathways and the function of downstream targets will aid in a better understanding of musculoskeletal pathologies such as chondrodysplasia, and the development of new approaches for the treatment of cartilage disorders. We will summarize recent findings, linking cell receptors and their potential signaling pathways to the control of chondrocyte behavior during early chondrogenesis and endochondral ossification.

Cellular homeostatic tension and force transmission measured in human engineered tendon

Tendons transmit contractile muscular force to bone to produce movement, and it is believed cells can generate endogenous forces on the extracellular matrix to maintain tissue homeostasis. However, little is known about the direct mechanical measurement of cell-matrix interaction in cell-generated human tendon constructs. In this study we examined if cell-generated force could be detected and quantified in engineered human tendon constructs, and if glycosaminoglycans (GAGs) contribute to tendon force transmission. Following de-tensioning of the tendon constructs it was possible to quantify an endogenous re-tensioning. Further, it was demonstrated that the endogenous re-tensioning response was markedly blunted after interference with the cytoskeleton (inhibiting non-muscle myosin-dependent cell contraction by blebbistatin), which confirmed that re-tensioning was cell generated. When the constructs were elongated and held at a constant length a stress relaxation response was quantified, and removing 27% of the GAG content of tendon did not alter the relaxation behavior, which indicates that GAGs do not play a meaningful role in force transmission within this system.

Photoresponsive Hydrogels with Photoswitchable Stiffness: Emerging Platforms to Study Temporal Aspects of Mesenchymal Stem Cell Responses to Extracellular Stiffness Regulation

An extensive number of cell-matrix interaction studies have identified matrix stiffness as a potent regulator of cellular properties and behaviours. Perhaps most notably, matrix stiffness has been demonstrated to regulate mesenchymal stem cell (MSC) phenotype and lineage commitment. Given the therapeutic potential for MSCs in regenerative medicine, significant efforts have been made to understand the molecular mechanisms involved in stiffness regulation. These efforts have predominantly focused on using stiffness-defined polyacrylamide (PA) hydrogels to culture cells in 2D and have enabled elucidation of a number of mechano-sensitive signalling pathways. However, despite proving to be a valuable tool, these stiffness-defined hydrogels do not reflect the dynamic nature of living tissues, which are subject to continuous remodelling during processes such as development, ageing, disease and regeneration. Therefore, in order to study temporal aspects of stiffness regulation, researchers have developed and exploited novel hydrogel substrates with in situ tuneable stiffness. In particular, photoresponsive hydrogels with photoswitchable stiffness are emerging as exciting platforms to study MSC stiffness regulation. This chapter provides an introduction to the use of PA hydrogel substrates, the molecular mechanisms of mechanotransduction currently under investigation and the development of these emerging photoresponsive hydrogel platforms.

A photoclickable peptide microarray platform for facile and rapid screening of 3-D tissue microenvironments

Microarrays are powerful experimental tools for high-throughput screening of cellular behavior in multivariate microenvironments. Here, we present a new, facile and rapid screening method for probing cellular behavior in 3D tissue microenvironments. This method utilizes a photoclickable peptide microarray platform developed using electrospun fibrous poly(ethylene glycol) hydrogels and microarray contact printing. We investigated the utility of this platform with five different peptide motifs and ten cell types including stem, terminally differentiated, cancer or immune cells that were from either primary origin or cell lines and from different species. We validated the capabilities of this platform to screen arrays consisting of multiple peptide motifs and concentrations for selectivity to cellular adhesion and morphology. Moreover, this platform is amenable to controlled spatial presentation of peptides. We show that by leveraging the differential attachment affinities for two cell types to two different peptides, this platform can also be used to investigate cell-cell interactions through miniature co-culture peptide arrays. Our fibrous peptide microarray platform enables high-throughput screening of 3D tissue microenvironments in a facile and rapid manner to investigate cell-matrix interactions and cell-cell signaling and to identify optimal tissue microenvironments for cell-based therapies.

The effect of decellularized matrices on human tendon stem/progenitor cell differentiation and tendon repair

It is reported that decellularized collagen matrices derived from dermal skin and bone have been clinically used for tendon repair. However, the varying biological and physical properties of matrices originating from different tissues may influence the differentiation of tendon stem cells, which has not been systematically evaluated. In this study, the effects of collagenous matrices derived from different tissues (tendon, bone and dermis) on the cell differentiation of human tendon stem/progenitor cells (hTSPCs) were investigated, in the context of tendon repair. It was found that all three matrices supported the adhesion and proliferation of hTSPCs despite differences in topography. Interestingly, tendon-derived decellularized matrix promoted the tendinous phenotype in hTSPCs and inhibited their osteogenesis, even under osteogenic induction conditions, through modulation of the teno- and osteolineage-specific transcription factors Scleraxis and Runx2. Bone-derived decellularized matrix robustly induced osteogenic differentiation of hTSPCs, whereas dermal skin-derived collagen matrix had no apparent effect on hTSPC differentiation. Based on the specific biological function of the tendon-derived decellularized matrix, a tissue-engineered tendon comprising TSPCs and tendon-derived matrix was successfully fabricated for Achilles tendon reconstruction. Implantation of this cell-scaffold construct led to a more mature structure (histology score: 4.08 ± 0.61 vs. 8.51 ± 1.66), larger collagen fibrils (52.2 ± 1.6 nm vs. 47.5 ± 2.8 nm) and stronger mechanical properties (stiffness: 21.68 ± 7.1 Nm m(-1) vs.13.2 ± 5.9 Nm m(-1)) of repaired tendons compared to the control group. The results suggest that stem cells promote the rate of repair of Achilles tendon in the presence of a tendinous matrix. This study thus highlights the potential of decellularized matrix for future tissue engineering applications, as well as developing a practical strategy for functional tendon regeneration by utilizing TSPCs combined with tendon-derived decellularized matrix.

Modulated crosslinking of macroporous polymeric cryogel affects in vitro cell adhesion and growth

Cell to matrix interactions affect in vitro cell adherence and proliferation and further decide the fate of tissue development for biomedical applications. This study demonstrates the role of crosslinking in altering the surface properties of 3D porous cryogel matrices. Glutaraldehyde and 1-ethyl-3(3-dimethylaminopropyl)-carbodiimide crosslinkers are used separately for the crosslinking of alginate and gelatin (i.e., AG(G) and AG(EN)), respectively. The difference in crosslinking affects the physiochemical properties of these matrices leading to variable cell behavior as demonstrated using four different cell types, which show homogeneous cell growth in AG(G) and spheroid cell growth in AG(EN). The present study shows successful use of controlled crosslinking in directing the cell growth for tissue engineering.